In order to maximize production from oil wells whose production has fallen below acceptable levels horizontal well bores or drainholes are sometimes drilled. The drilling operation typically is carried out by milling a section out of the casing in the area to be drilled and deflecting the drill at a predetermined angle into the wall of the vertical bore by means of a whipstock positioned just below the juncture of the existing vertical bore and the horizontal bore to be drilled. The drill will enter the surrounding formation at a relatively shallow angle to the vertical bore and is moved along an arcuate path which may terminate at approximately the elevation corresponding to the point of entry of the drill.
It is sometimes necessary to log the horizontal bore prior to production operations to determine information about the intersected formations. It can also be important to obtain pressure and temperature data from the horizontal drainhole after operations have begun in order to determine the cause of lower than anticipated recoveries. For example, if it is suspected that cross flow of oil and gas is occurring between individual fracture compartments intersected by the horizontal drainhole, pressure and temperature readings can be used to determine whether the cross flow is in fact occurring and where. With this information compartments of maximum oil flow can be isolated with lateral hole production packers and the isolated compartments can then be produced.
Conventional wireline techniques are inapplicable in logging horizontal drainholes because it is extremely difficult or impossible to guide a sensor supported only by a flexible wireline into and along the arcuate path of the drainhole. If the radius of the arcuate path of the horizontal drainhole is relatively long, for example, greater than about 10 meters, it is likely that conventional tubing-conveyed techniques can be used to move the sensor through the drainhole. Thus a tube string carrying a sensor at its end is normally capable of entering such a drainhole and traversing its gentle arc without becoming snagged or stuck. When the radius of the drainhole is short, for example less than about 10 meters, the problem of moving the sensor into and through the drainhole becomes much more difficult.
A variety of ways to move a sensor through a non-vertical bore hole have been suggested. Self-propelled sensor carriages developed for use in non-vertical holes are not suited to travel over the sharply curved path of the type of horizontal drainhole under discussion and, moreover, they are too expensive for the relatively short logging operation contemplated. Another suggestion is to provide a flexible sensing means which can be caused to move through a horizontal bore by pressurized fluid. This design, however, also is unnecessarily complicated for the contemplated use and it too is expensive due to the special fabric construction required.
Although is may be possible to design other specialized equipment to carry out this specialized function, the attendant high costs and the delays caused by the need to design and construct new equipment for each new situation would make such new designs undesirable. Ideally, the equipment for logging a short radius horizontal drainhole should make use of existing components, be simple in design, and be easy and inexpensive to fabricate. Prior to this invention such equipment was not known.